1. Field of the Invention
The present invention is generally related to angular position sensors and, more specifically, to a method for calibrating the magnetization of a permanent magnet which is needed to achieve certain specified output characteristics in response to the angular movement of the permanent magnet and the shaft to which it is attached.
2. Description of the Prior Art
Many different types of angular position sensors are known to those skilled in the art. Alfors U.S. Pat. No. 5,164,668, which issued on Nov. 17, 1992, discloses an angular position sensor which exhibits decreased sensitivity to shaft position variability. The angular position sensor is provided with first and second pole pieces that extend from regions proximate a rotatable magnet to regions proximate a magnetically sensitive device. The pole pieces provide defined magnetic paths of lowered reluctance that confine the lines of flux extending between the rotatable magnet and the magnetically sensitive device. The placement of the rotatable magnet between first and second pole piece segments of the invention significantly reduces the sensitivity of the sensor to variations in position of the rotatable magnet and therefore increases the reliability of the measurement system. This reduced sensitivity inhibits the degradation of operational accuracy that could otherwise be caused by inaccuracies in the magnet's shaft position, large tolerances in the dimensions of the shaft diameter and the bearing diameter and variable location of the shaft because of excessive wear. U.S. Pat. No. 5,164,668 is explicitly incorporated by reference in this application.
Wolf et al U.S. Pat. No. 5,332,965, which issued on Jul. 26, 1994, describes a linear angular position sensor which has an adjustable flux concentrator for sensitivity adjustment and temperature compensation. The position sensor includes a magnetically sensitive device, such as a Hall effect element, and a plurality of flux concentrators rigidly disposed relative to the Hall effect element. The assembly is disposed in a housing a fixed distance from a rotatably mounted standard magnet defining a fixed airgap therebetween. The magnet is disposed in a rotatably mounted magnet holder which also acts as a drive arm that is adapted to be mechanically coupled to a pivotally mounted device. The configuration of the flux concentrators assembled to the magnetically sensitive device causes the output of the Hall effect element to be generally linear. In order to avoid problems associated with electrically adjustable angular position sensors, the angular position sensor is adjusted mechanically. In particular, a flux concentrator, preferably having a halo shape, is disposed adjacent the magnet. The sensor is calibrated by varying the distance between the halo-shaped flux concentrator and the magnet. In one embodiment of the invention, the halo-shaped flux concentrator is formed to provide temperature compensation for the sensor. The sensor is hermetically sealed and is thus unaffected by wear or vibration.
U.S. patent application Ser. No. 08/405,651, now U.S. Pat. No. 5,512,820, which was filed on Mar. 17, 1995 by Alfors discloses a rotational position sensor with a two-part rotatable member to resist jamming. The rotational position sensor is provided with a rotatable member that comprises first and second portions. The first portion is generally cylindrical and has an opening that is shaped to receive the second portion therein with a resilient spring disposed in the annular gap between the first and second portions when this assembly is accomplished. A permanent magnet is molded into or otherwise affixed to an extension of the first portion and the second portion of the rotatable member is shaped to receive a shaft. Relative rotation is permitted between the first and second portions of the rotatable member so that the shaft will not be seized in position if the rotatable member is jammed within a stationary portion of the sensor. If jamming occurs, such as that which might be caused by icing, the first portion of the rotatable sensor might be seized in position and restricted from further rotational movement. However, the second portion will remain free to rotate with the shaft because of the relative angular movement that is permitted between the first and second portions. This movement is in association with the changing torsion of the spring disposed in the gap between the first and second portions. When the seizing of the first portion of the rotatable member is eliminated, the first and second portions of the rotatable member move back to their regular positions relative to each other and continued normal operation is possible.
Both of the devices described immediately above utilize a rotatable permanent magnet that is attachable to a shaft. Rotation of the permanent magnet about a centerline of the shaft allows the sensor to determine the angular position of the shaft with respect to a preselected reference position. A magnetically sensitive component is disposed in magnetic association with the permanent magnet, typically through the use of ferromagnetic pole pieces, and an output of the magnetically sensitive component is used to represent the angular position of the permanent magnet and its attached shaft.
Several problems exist with regard to the calibration and testing of angular position sensors of the type described immediately above. Since permanent magnets can vary from one to another in their magnitude and precise direction of magnetization, the output signal from the magnetically sensitive component may exhibit a characteristic in relation to the angular position of the permanent magnet that is not the precisely desired characteristic. In addition, because the variability of manufacture can result in one angular position sensor being slightly different in its magnetic characteristics than another, the intensity of the magnetic field imposed on the magnetically sensitive component may differ from sensor to sensor for the same angular position of the permanent magnet. Because of this, several techniques have been developed that allow angular position sensors to be calibrated to account for these variabilities. One calibration technique involves the use of thin film resistors that can be trimmed with a laser to adjust the output signal from the magnetically sensitive component. Other techniques may involve the mechanical movement of ferromagnetic objects to advantageously affect the magnetic field. During calibration, these ferromagnetic objects are typically moved until the output signal from the magnetically sensitive component matches a desired output signal for each of several preselected angular positions of the permanent magnet. Both of these calibration techniques involve the use of components (e.g. circuits, magnets, pole pieces, magnetically sensitive elements) that combine to result in an initial output signal from the magnetically sensitive component which is later adjusted or corrected to make the output signal characteristic exhibit a preselected and desired characteristic in relation to the angular position of the permanent magnet and its attached shaft. In other words, through the use of either electrical or mechanical alteration, the assembled angular position is altered to provide a correct output characteristic of the magnetically sensitive component's output signal as a result of the angular position of the shaft. It would therefore be advantageous if a calibration procedure could be developed which allows the magnet to be magnetized in such a way that it compensates for all other variabilities of the sensor to achieve a desired characteristic of the output signal from the magnetically sensitive component as a function of the angular position of the permanent magnet.